1. Field of the Invention
This invention relates to a high-voltage metal oxide semiconductor device and a fabrication method thereof, and more particularly relates to a high-voltage metal oxide semiconductor device with a vertical well and a fabrication method thereof.
2. Description of Related Art
Among various power semiconductor devices, the metal oxide semiconductor field effect transistor, with the advantages of fast switching, low switching loss, and low driving loss, has been widely used for high-frequency power conversion. However, it is hard for the traditional power semiconductor device to withstand high voltage. In order to enhance withstanding voltage, on-resistance of the power semiconductor may increase disproportionately, which results in huge conduction loss and also seriously restricts the application of the power semiconductor device.
Referring to FIGS. 1 and 1A, on-resistance of the traditional high-voltage semiconductor field effect transistor (RDS(on)) is dominated by the resistance of the drift zone, which includes Rch, Ra, and Repi as shown. The voltage blocking capability of the high-voltage semiconductor field effect transistor is mainly decided by the distance of the drift zone and the doping. That is, in order to increase withstanding voltage, the epitaxial layer should be thickened and the doping concentration should be lightened. However, the thickened epitaxial layer and the lightened doping concentration results in disproportionate increasing of on-resistance.
The percentage of the epitaxial layer contributed to the overall on-resistance varies with the withstanding voltage. As shown, for the metal oxide semiconductor designed to withstand the voltage (VGD) of 30V, the epitaxial layer contributes only 29% of the total on-resistance, whereas, for the metal oxide semiconductor designed to withstanding the voltage (VGD) of 600V, the epitaxial layer contributes 96.5% of the total on-resistance.
There are two typical methods to reduce the total on-resistance of the high-voltage metal oxide semiconductor device. The first one is to increase the cross-section area of the transistor so as to reduce on-resistance crossing the epitaxial layer. However, the integration density must be reduced and the cost is increased. The other one is to introduce minority carriers. However, this method not only slow down the switching speed but also result in the existence of tail current that increases switching loss.
Since both the above two methods have the unsolvable drawbacks, it is eager to find out a new high-voltage metal oxide semiconductor device with both low on-resistance and high voltage blocking capability.